In its new, lower orbit, NASA's Magellan spacecraft has
acquired high-resolution gravity data over about one-third of
Venus by measuring the motions of the orbiting spacecraft through
its radio signal.

The gravity data give scientists a glimpse inside Venus --
revealing the internal structure that lies below its surface --
by disclosing the relative density of different parts of the
planet.

"These new data provide high resolution in the polar regions
which previously was very poor due to the orbit's very high
altitudes," William L. Sjogren, head of the Magellan Project's
gravity team at NASA's Jet Propulsion Laboratory, told the 1993
fall meeting of the American Geophysical Union in San Francisco.

Sjogren's presentation was one of several given at the AGU
meeting by gravity team members and other Magellan scientists.

On August 6, the spacecraft successfully completed a 70-day
aerobraking phase which changed its orbit from highly elliptical
to nearly circular and began high-resolution gravity mapping.

Two major areas of study on Venus, the Lakshmi and Maxwell
Montes regions, were resolved for interior structure, Sjogren
said.

Maxwell, a far northern mountain range, is the highest
region on Venus. Sjogren said it is critically important to
geophysicists who need to know the gravity field to realistically
infer Venus' internal structure.

The mantle of Venus, as on Earth, is believed to consist of
dense, rocky material that flows slowly in response to internal
heating and brings heat energy from deep in the planet to the
upper regions. Gravity observations can be used to identify
regions of hot, upwelling mantle.

The Magellan spacecraft went into orbit around Venus in
August 1990, and performed surface mapping of 98 percent of the
planet with imaging radar for two years. In its fourth 243-day
orbiting cycle, it began gravity mapping using only its radio
signal.

A 243-day cycle represents one rotation of the planet
beneath the spacecraft.